1. Field of Invention
The present invention relates to a compression refrigeration system including a compressor, a heat rejector, an expansion means and a heat absorber connected in a closed circulation circuit that may operate with supercritical high-side pressure, using carbon dioxide or a mixture containing carbon dioxide as the refrigerant in the system.
2. Description of Related Art
Conventional vapour compression systems reject heat by condensation of the refrigerant at subcritical pressure given by the saturation pressure at the given temperature. When using a refrigerant with low critical temperature, for instance CO2, the pressure at heat rejection will be supercritical if the temperature of the heat sink is high, for instance higher than the critical temperature of the refrigerant, in order to obtain efficient operation of the system. The cycle of operation will then be transcritical, for instance as described in WO 90/07683. Temperature and high-side pressure will be independent variables, contrary to conventional systems.
WO 94/14016 and WO 97/27437 both describe a simple circuit for realising such a system, comprising a compressor, a heat rejector, an expansion means and an evaporator connected in a closed circuit. CO2 is the preferred refrigerant for both systems.
The system coefficient of performance (COP) for transcritical vapour compression systems is strongly affected by the level of the high side pressure. This is thoroughly explained by Pettersen & Skaugen (1994), which also presents a mathematical expression for the optimum pressure. Because high side pressure is not a function of temperature, high side pressure can be controlled in order to achieve optimum energy efficiency. To do so it is necessary to determine optimum pressure for given operating conditions.
Several publications and patents are published which suggest different strategies to determine the optimum high side pressure. Inokuty (1922) published a graphic method already in 1922, but it is not applicable for the present digital controllers.
EP 0 604 417 B1 describe how different signals can be used as steering parameter for the high side pressure. A suitable signal is the heat rejector refrigerant outlet temperature. The correlation between optimum high side pressure and the signal temperature is calculated in advance or measured. Densopatent describes more or less an analogous strategy. Different signals are used as input parameters to a controller, which based on the signals regulates the pressure to a predetermined level.
Among others, Liao & Jakobsen (1998) presented an equation which calculates optimum pressure from theoretical input. The equation does not take into account practical aspects which may affect the optimum pressure significantly.
Most methods for optimum pressure determination described above take a theoretical approach. This means that they are not able to compensate for practical aspects like varying operating conditions, and the influence of oil in the system. Optimum pressure is thus frequently different from the calculated one. There is also a risk for a “wind up” and lack of control. This happens when a temperature signal gives a feedback to the controller, which adjust the pressure with some delay. If conditions change rapidly, the controller will never establish a constant pressure, and cooling capacity will vary.
As explained above, it is a possibility to run tests and measure optimum high side pressure relations. But this is time consuming and expensive. Furthermore, it is hard, if not impossible, to cover all operating conditions, and the measurements have to be performed for all new designs.